This work presents a model for the simulation of transverse mode instability (TMI) in rare earth doped optical fiber amplifiers. The model evaluates the internal temperature of a fiber using a superposition of a finite number of thermal eigenmodes. This simplification greatly enhances the speed of calculation with negligible impact on calculation accuracy. This new method is described and quantitatively compared to an older model that uses standard, spatially resolved FDTD to integrate the heat diffusion equation. When tested over a range of spatial and temporal resolutions, this model reduces runtime by a factor of ∼13.9 on average relative to identical simulations using the spatially resolved model.

Undetected contamination can cause catastrophic damage to optical components in high average power laser systems with potential for additional damage to optical mounts and contamination of nearby optics, causing unexpected downtime and requiring costly repairs. Such particle-induced damage depends strongly on the characteristics of both the contaminant material and the underlying coating or substrate. This work analyzes the effects of carbon-based contaminant particles on the high average power laser-induced damage threshold (LIDT) of optics, specifically the effects of particle size and concentration. To develop a theoretical model that can predict particle-induced LIDT, experimental measurements of CW laser damage were taken for contaminant particle sizes ranging from 0.5-40μm with concentrations ranging from ∼0.6 mm ⁻² to ∼400 mm ⁻² . Simple theoretical arguments can explain many of the qualitative results, and a quantitative free-carrier absorption model that incorporates contaminant material evaporation, diffusion, redeposition, and oxidation closely fits the experimental data. This model has been used to draw general conclusions about safe contamination levels, specifically that systems using Ta ₂ O ₅ /SiO ₂ reflectors at an irradiance greater than 100kW/cm ² should strictly avoid carbon-based contaminants with diameters greater than approximately ∼12.3 μm . Local contaminant concentrations with a fill factor greater than ∼0.5% are also likely to cause damage, regardless of contaminant particle size.

Multiwavelength adaptive optic systems can experience phase errors when corrections measured at one wavelength are applied to others. Such errors originate from several sources, but poor optical coating design can play a major role since the phase dependence of any coating varies greatly with wavelength, angle, and thickness. To address these errors, a multilayer coating design is proposed where the effective electric field penetration depth into the coating is similar at all design wavelengths. Using this method, a two-wavelength coating is designed for 1070 and 1550 nm and compared to a standard two-wavelength high-reflectivity coating consisting of stacked Bragg reflectors. The standard stacked Bragg reflector design induces up to 173.0 nm of phase error across the wavelength band that reflects off the more deeply buried part of the multilayer. Alternatively, the two-wavelength equal field penetration design maintains a high reflectivity of 99.999%, while only inducing phase variations of 28.6 nm for both target wavelengths.

In multiwavelength adaptive optic systems, optical coatings contribute significantly to phase correction inconsistencies for different wavelengths, incident angles, and locations within a beam. Theory has shown that optical coatings can be specially designed to correct for these errors. In this paper, phase-corrected optical coatings for multiwavelength adaptive optics have been designed, deposited, and characterized, demonstrating an over 80% reduction in phase differential across the optic compared to standard coating designs. Two coating stack designs were utilized for comparison, with both reflecting highly at two wavelengths, 633 and 1064 nm: the first was a standard stack of two Bragg reflectors for those wavelengths, and the second was a phase-optimized design that forced the two wavelengths each to reflect with similar penetration depths into the optic. The coatings were composed of silicon dioxide and silicon nitride deposited by PECVD. Each was deposited with an intentional thickness variation of near 1% across the optic to highlight the intrinsic differential phase variation across each coating at each wavelength. The coated optics were then characterized for reflectivity and wavefront. The stacked Bragg design showed a differential phase variation across the optic of 0.54 radians between the two wavelengths, whereas two separate samples of a phase-optimized design showed dramatically decreased corresponding phase differentials between wavelengths of 0.04 and 0.10 radians. The coatings were then sectioned and measured by a SEM to ensure they met the expected layer thickness values, which, when input into a coating optimization simulation, indicate that the optimized coating phase performance is within 0.05% of the multiwavelength adaptive optic theoretical design predictions.